Metchnikoff thought that this microbe, or some choleriform bacillus, similar though not specific, probably served as a natural vaccine against cholera in those localities which were spared by the epidemic though the cholera vibrio was brought there. This was a question that could only be solved by experiment.
At the time when he had himself absorbed a cholera culture, Metchnikoff admitted the risk of catching the disease; still, his eagerness to solve the problem had silenced in him all other considerations and feelings opposed to his irresistible desire to attempt the experiment. This "psychosis," as he himself called it later, recurred now, in spite of all the emotions he had gone through on the previous occasion, and he decided once again to experiment on man. It is true that he now only had to deal with choleriform microbes from Versailles which he believed to be quite harmless as they came from the water of a locality free from cholera. He therefore ingested some of the Versailles choleriform vibriones and gave some to several other people. Contrary to expectation, one of the latter, an incurable epileptic, showed some symptoms of cholera, but recovered. But as, a short time later, this patient died from a cause which remained obscure, Metchnikoff thought that possibly the experiment might have had something to do with it, and finally resolved to perform no other experiments on human beings.
How could that unforeseen result be explained? Metchnikoff supposed that the intestine of the subject contained favourable microbes which had exalted the virulence of the bacillus, in itself weak and innocuous. If it were so, then certain intestinal microbes would influence the inception of diseases and the action of the micro-organisms would vary according to the society in which they found themselves. As such problems could only be solved through experiment, he again energetically sought for a means of conferring cholera upon animals. After many failures and difficulties, it occurred to him to try new-born animals whose intestinal flora, not yet developed, could not interfere with the swarming of the ingested bacilli. He chose young suckling rabbits for his experiments and, with the aid of _favourable_ microbes, he succeeded at last in giving them characteristic cholera, through ingestion; thus it became possible to study intestinal cholera on these animals.